1. Field of the Invention
This invention relates to an optical transmission system using wavelength division multiplexing (WDM) techniques, and more particularly to a wavelength division multiplexing optical transmission apparatus and an optical repeater used therewith.
2. Description of the Related Art
With the recent advances in optical fiber amplifiers, tremendous research effort has been directed toward long-distance, large-capacity transmission. In such research activities, a wavelength multiplexing optical transmission system has been attracting attention as a very attractive system because it is capable of increasing the overall transmission capacity remarkably by multiplexing optical signals in the wavelength region without increasing the transmission capacity per channel.
When optical signals are multiplexed very density on an axis of wavelength (or frequency), fluctuations in the wavelength of the transmitter and the wavelength characteristics of the optical wavelength multiplexer/demultiplexer result in the deterioration of receiver sensitivity. Therefore, monitoring the wavelength throughout the entire system including the transmitter and receiver is an essential technique.
The monitoring of wavelength of the transmitter has been effected by monitoring the operating temperature, injection current, and output power of a semiconductor laser used as a light source. Only these pieces of monitoring information are insufficient to cope with the deterioration of a semiconductor laser with age.
To overcome this problem, a method has been proposed which uses an optical resonator as a wavelength reference unit to monitor the wavelength of the semiconductor laser and performs feedback control of the injection current and operating temperature to stabilize the wavelength of the semiconductor laser (e.g., as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-15992). In such wavelength stabilization, the output beams of the semiconductor lasers are multiplexed by an optical coupler, which then transmits a wavelength division multiplex optical signal to an optical fiber and couples part of the signal to a Mach-Zehnder interferometer. On the basis of the output light from the Mach-Zehnder interferometer, the wavelengths of the semiconductor lasers are controlled in unison.
With such wavelength stabilization, however, because no measures have been taken against the instability of wavelength stabilizing control, the deviation of the wavelength of the transmitter due to the unstable control will degrade the receiver sensitivity. In wavelength division multiplexing transmission, an optical filter for demultiplexing the individual wavelengths is essential to the receiver section and the stability of the filter's wavelength characteristic is very important in terms of receiver sensitivity. Taking these things into consideration, the approach has been proposed of controlling the transmission wavelength characteristic of an optical filter so that the received power may be maximal after the demultiplexing at the optical filter (e.g., as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-222237).
With the conventional method of controlling the transmission wavelength characteristic of an optical filter, fluctuations in the wavelength of transmitters causes the transmission wavelength characteristic of the optical filter to fluctuate, which results in an increase in the crosstalk between channels, causing the problem of degrading the receiver sensitivity. Furthermore, with the conventional wavelength division multiplexing transmission apparatus, since no measures have been taken against the instability of wavelength stabilizing operation at the transmitter section, the deterioration of the receiver sensitivity will occur when the wavelength control becomes unstable.
Furthermore, since the transmission wavelength characteristic of the optical filter that demultiplexes the wavelength division multiplex optical signal for each wavelength is stabilized to the wavelength of the transmitter at the receiver section, the deviation of wavelength of the transmitter would cause the crosstalk between channels to increase, leading to the deterioration of receiver sensitivity.
Furthermore, with the conventional wavelength division multiplexing apparatus, in stabilizing the wavelength of the semiconductor laser, all of the wavelengths of the semiconductor lasers are controlled in unison using an optical element, such as a Mach-Zehnder interferometer, as a wavelength reference, causing the problem that the wavelength capture range is limited to less than the channel spacing.
Furthermore, since the wavelength characteristic of such an optical element as a Mach-Zehnder interferometer depends on temperature, it is difficult to provide stable wavelength control because of the influence of ambient temperature change. In addition, since a conventional semiconductor laser presents a very small frequency modulation efficiency in the frequency range of several kHz to several hundred kHz, the frequency modulation does hot work well in that range, leading to the problem of being unable to stabilize the wavelength.
Furthermore, with conventional optical repeaters that amplify wavelength division multiplex optical signals, the gain per channel of optical fiber amplifier differs, depending on the number of optical signals multiplexed, causing the problem of degrading the receiver sensitivity.